This invention relates to pressure relief or back pressure valves. Pressure relief or back pressure valves in which a flexible diaphragm is resiliently urged against a valve seat are well known. It is often necessary in fluid pressure systems in which such valves are used to bleed air from the system.
It is therefore an object of the invention to provide a pressure relief or back pressure valve which, in addition to its normal function, can be actuated to enable air to be easily bled therethrough when necessary.